The present invention relates to a tappet used in a direct-acting valve operating mechanism to decrease frictional resistance with a cam.
In a direct-acting valve operating mechanism of an internal combustion engine, various tappets are used, such as a cylindrical steel tappet and a cylindrical Al tappet which has a bore on the upper surface with which a metal shim is engaged.
Recently, in view of preservation of earth environment, control of exhaustion of CO2 has become strict. In automobiles, it is strongly required to improve fuel economy.
To improve fuel economy in automobiles, mechanical loss of an engine, especially friction loss in a valve-operating mechanism, may be preferably decreased as much as possible.
In a direct-acting valve operating mechanism in which the upper surface of a tappet is pressed by rotation of a cam to drive a poppet valve directly, sliding frictional resistance between the upper surface of a tappet and a cam in low- and middle-rotation-speed range becomes larger. To decrease frictional resistance, it is preferable to lubricate the interface between the upper surface of a tappet and a cam, but in a conventional tappet, the upper surface is formed as flat to decrease oil-storage properties of lubricating oil, so that it is difficult to decrease frictional resistance.
In particular, in a V-type engine in which an axis of a tappet is inclined, oil-storage properties of lubricating oil on the upper surface of a tappet is further decreased to increase frictional resistance with the cam, so that wear with the interface is increased.
A known steel tappet is heavy, which decreases output and fuel economy. A known Al tappet is light, but is expensive in manufacturing.
In view of the disadvantages, it is an object of the invention to provide a tappet for an internal combustion engine, the tappet being light and inexpensive to lubricate the upper surface of a top wall effectively and decrease frictional resistance with a cam.
According to the present invention, to achieve the object, there is provided a tappet for an internal combustion engine, comprising:
a top wall which is engaged with a cam; and
a cylindrical portion into which a poppet valve is inserted, a downward projection being formed in the middle of the lower surface of the top wall and having an oil-storage recess on the upper surface, diameter of the projection being smaller of width of the cam.
By the lubricating oil stored in the recess, frictional or wear resistance is improved between a cam and the upper surface of the tappet, and/or between the lower surface of the projection and the upper end of a poppet valve which moves up and down.